Machine tools of the type having a tool storage magazine are conventionally used in the field of metal machining. In machine tools of this type, engagement of a tool to be used in a machining process with the spindle structure mounted on the machine tool is performed prior to the start of the machining process, while disengagement of the tool from the spindle structure is performed as soon as the machining process is completed.
In typical machine tools of the type described above, a tool engaging process is adopted in which, when one of the tools in the tool magazine must be engaged with the spindle of the spindle structure, the axis of the tool is first brought into axial alignment with the axis of the spindle of the spindle structure. Thereafter, an axial movement between the spindle head of the spindle structure and the tool magazine is made until the tool approaches the spindle and is eventually engaged with the spindle. Further, when it is required to return the tool from the spindle to the tool magazine after completion of the machining process, a tool disengaging process for disengaging the tool from the spindle of the spindle structure is employed.
In the above-described conventional machine tools, transmission of a rotating drive force from the spindle to a tool engaged with the spindle is effected by adopting a mechanical structure in which a tapered portion in a tool arbor and a corresponding tapered bore formed in the tool receiving portion of the spindle are tightly fitted to one another. The adoption of the above-described mechanical structure is effective only in the case where the tool received by the spindle is used for effecting a simple drilling process of a metal workpiece. However, in the case where the above-described mechanical structure is adopted for a tool which is used for effecting a milling process of a workpiece, the tool is loosened from the spindle due to a force acting on the tool in the direction perpendicular to the axis of the tool during the milling process. Consequently, the accuracy of the milling process is decreased, and this lower accuracy is unfavorable from the point of view of mechanical performance of a machine tool.
In order to prevent a decrease in the accuracy of the milling process, some conventional machine tools employ a spindle structure in which a draw-bar is provided inside the spindle to enable the spindle to firmly hold a tool. However, in the spindle structure of a machine tool having a draw-bar, a particular draw-bar actuator, such as an air cylinder or a hydraulic cylinder, must be arranged to actuate the draw-bar so as to loosen a tool from the spindle when the tool is to be transferred from the spindle to the tool magazine. Since an actuator, such as air cylinder or hydraulic cylinder, is expensive its use results in the production cost of the entire machine tool being too high.